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Harmonics demystifiedTim Makris, Danfoss Australia
ABsTrACTThisarticleprovidesaquickoverviewofharmonicsinelectricalpowernetworks,thecauses,themitigationmethodsandapplicablestandards.Thearticlealsodiscussesbestpracticeapproachtoachievetheoptimumbalancebetweencost/performanceformoderninstallationswithintheHVACindustry.
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inTroDuCTionHarmonicsarefairlycommoninmodernelectricalinstallationsandformonepartofameasurementofpowerquality.Equipmentsuchascomputerpowersupplies,lightingwithelectronicballasts,UPSsystemsandvariablespeeddrives(VSD)distortthecurrentdrawnfromthenetwork,causeharmonicsanddisturbotherconsumersorotherequipmentconnectedtothesamesupplynetwork.Harmonicdistortion,ifnotmitigated,canleadtoincreasedconsumptionofpower,reducedpowersystemutilisation,increasedlossesincablesandtransformersandequipmentmalfunction.
Mitigationforharmonicscanbeappliedatvariouspointswithintheelectricalnetworkofabuildingdependingonthesourceofthedistortion,thelocationandpowerratingofthesourceofthedistortionrelativetothesupplycapacity.Filteringcanbeappliedattheterminalsofthedevice,atanMCCorswitchboardfeedinganumberofdistortingloadsoratthetransformer.Agoodanalogyistoconsiderapipingsystem,withthewaterbeingthepowersupplyandharmonicsacontaminantwithinthewater.Themitigatingfilterisafilterthatremovesthecontaminantfromthewatersupply.Thelocationofthefilterthendetermineshowfarwithinthebuilding’snetworkthecontaminant,theharmonics,areallowedtoflow.
Whateverharmonicsareunfilteredwillalsofindtheirwayintotheelectricalnetworkfeedingotherconsumersofelectricity.Tomaintaincompatibilityandareasonablelevelofpowerquality,standardsexistlimitingtheamountofdistortioneachconsumerisallowedtoproduce.Electricaldistributorshavepowertoimposespecificlimitstoeachconsumerorbuilding.
Asthelocationandpowerratingoftheharmonicproducingequipmentdeterminesthemethodoffiltering,sotoothetypeoffilterandthelocationwithinthenetworkwilldetermineoverallcostandimprovementtothebuilding’selectricalnetwork.Whichfilterisusedandwhereitisusedwillalsogovernwhethercompliancewithstandardsorlimitsimposedbyanelectricaldistributororaspecificationbyaconsultantcanbemet.
WHAT Are HArMoniCs?Theidealsupplyvoltagewaveformisasinewave.InAustraliathevoltageis230V(+10%,–6%)withafrequencyof50Hzforsingle-phasesupplies,and400V/50Hz(+10%,–6%)forthree-phasesupplies.AnexampleoftheperfectsinewaveisshowninFigure1.
Figure 1: Perfect single-phase sine wave
Whenaload(equipment)isconnectedtotheelectricalsupply,currentisdrawnbytheload.Inthecontextofharmonics,loadscanbelistedaseitherlinearornon-linear.Theloadiscategorisedbythecurrentitdraws.Forexample,incandescentlightbulbs,resistanceheaters,directon-line(DOL)connectedmotorsarelinearloads.Thecurrentdrawnbylinearloadsisasinewave,althoughmaybeoutofphasewiththevoltagesupply(displacementpowerfactor).
Non-linearloadsconsistofcomponentsdrawingnon-sinusoidalcurrent.Single-phaserectifiers(powersupplies),lightingwithelectronicballasts(fluorescentlights),VSD,alldrawnon-sinusoidalcurrent.Figure2showstypicalcurrentfromasingle-phaserectifier[1].Figure3showsatypicalcurrentwaveformfromathreephaseVSDwithoutanyharmonicfiltering[1].
Figure 2: Typical single-phase rectifier input current waveform
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Figure 3: One phase of a three-phase VSD waveform without harmonic filtering
Thesedistortedwaveformsarerepetitiveandconsistofcomponentwaveformsatdifferentfrequencies(harmonics)ofthefundamental50Hzwaveform.Thesecomponents,orharmonics,combinedproducetheresultantwaveformsshowninFigures2and3.
Figure 4: Three phase VSD harmonic spectrum without harmonic filtering
Acommonwaytodisplayharmonicsisasaspectrum(frequencydomain).Figure4[1]showsthespectrumoftheVSDwaveforminFigure3.Eachfrequencyisshownasabarwithaparticularmagnitudeasapercentageofthefundamental,50Hz.
Theharmonicvaluesarecombinedtoprovideasinglevaluethatcanquantifytheamountofharmonicspresent–totalharmonicdistortion,orTHD.HarmonicsinbothcurrentandvoltagecanbeexpressedasTHD:THDIandTHDVrespectively.TheformulaforTHDisexpressedas:
WhereI(h)isthecurrentatthecomponentharmonicnumbers(forexample5th,7th,etc.)I(1),isthefundamentcurrent.ThewaveforminFigure3hasaTHDofapproximately104%.
THe AffeCT of HArMoniCsIfthenon-linearcurrentdrawnbytheloadisconsumedbytheloadandonlytheload,thenwhatistheproblem?Theproblemofdistortedcurrentisthatitcreatesadistortedvoltageinthesupplythatisseenbyotherequipmentorconsumersconnectedtothesamesupply.
Thesupplynetworkconsistsofanimpedance.Thesource,thetransformer,andthecableallhaveimpedances.Whencurrentwithharmonicdistortionisdrawnthroughthesupplyimpedance,avoltagedropiscreatedateachparticularharmonicfrequency.ThesevoltagedropsdistortthevoltagewaveformfromtheidealsinewaveasseeninFigure5.
Figure 5: Distorted voltage waveform
Bothharmoniccurrentandvoltagedistortionaffectthenetworkandotherequipmentorconsumers.Asthenon-linearloadcreatescurrentharmonics,theseharmonicsincreasetheRootMeanSquared(RMS)currentoftheload–averagecurrentovertime.TheincreaseinRMScurrentduetoharmonicseffectthesupplybycausingincreasedthermallossesinsupplytransformersandgenerators.Thehigherfrequenciesoftheharmonicsalsoincreaselossesintransformersandgeneratorsbyincreasingtheeddycurrentlosses.Theseincreasedlossesaredissipatedasheat,raisingthetemperatureofthetransformer,itswindingsandinsulation,decreasingitslifetime.
Harmonicsalsohaveaneffectoncablesthatconductdistortedcurrent.AswiththeeffectontransformerstheharmoniccurrentsincreaseRMScurrentandthereforeincreasecablelosses.Anadditionaleffecttheharmonicfrequencieshaveoncablesisknownaskineffect.Skineffectdescribesthephenomenonofhowdifferentfrequenciesofcurrentaredrawnwithinaconductor.Thefundamentalfrequencycomponentofcurrent(currentat50Hz)isdistributeduniformlyacrossthecross-sectionoftheconductor.Theharmonicsathigherfrequencies,typicallyabove500Hz,aredistributedclosertotheouterlayersoftheconductor,reducingtheeffectivecross-sectionalareaandincreasingthecableresistanceandincreasingthermallossesincable.
Singlephasenon-linearloadssuchascomputerpowersuppliesandelectroniclightballast,createharmonicsknownastriplens–multiplesofthethirdharmonic.Thesetriplensaddupintheneutralconductorofthesupply,placingextraloadontheneutral.Itiscommonpracticetooversizeneutralconductorstoaccommodatetheadditionalharmonicloadingsingle-phasenon-linearloadsgenerate.
ExcessiveharmonicvoltagedistortioncancausetorquepulsationsandincreasedlossesinDOLconnectedmotors.Typically,a10°Criseintemperatureabovetheratedtemperaturecanreducemotorinsulationlifeupto50%.Thehighfrequencyharmonicspresentinadistortedvoltagewaveformcauserotatingfluxwithinthemotoratfrequenciesabove50Hz,whichinteracttocreatetorqueopposedtothenormal,ratedtorqueoutput,leadingtoshaftpulsations.
10
20
40
60
80
100
3 5 7 9 11 13 15 17 19 21 23 25
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% F
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CircuitbreakersaredesignedtomonitorRMScurrentandpeakcurrent.AdistortedcurrentwaveformhashigherRMSvaluesandpeakvaluesaboveastandardsinewave.Thecircuitbreaker,ifnotcapableoftrueRMSmonitoringmayneedtobeadjustedtoaccommodateharmonicsifnuisancetripsoccur.Similarly,conventionalmeteringequipmentcanalsoindicateincorrectvalueduetoharmonics.MeteringequipmentcapableoftrueRMSreadingsisrecommendedifmeteringofnon-linearloadsisrequired.
Powerfactorcorrectioncapacitorscanbesusceptibletoproblemsifconnectedonasupplywithsignificantlevelsofharmonicspresent.Capacitorsabsorbharmoniccurrentsand,ifnotfilteredordimensionedfortheharmoniccurrents,canbedestroyed.Resonancecanalsooccurwhenthecapacitor,combinedwiththesupplyimpedanceattheharmonicfrequencies,magnifytheamplitudeofharmonicscurrentsandvoltagesleadingtodamagetoplantandequipment.De-tuningreactorsarenecessarytoprotectthepowerfactorcorrectioncapacitors.
Thepowerfactoroftheinstallationisalsoaffectedbyharmonicproducingnon-linearloads.Non-linearloadshaveadisplacementpowerfactorclosetounity.However,thetruepowerfactoralsofactorsharmonics.Theformula(2)hastheapparentpowerS(thepowerdeliveredtotheequipment),realpowerP(thepowerrequiredtodowork),reactivepowerQandharmonicpowerD.
Assumingalmostunitydisplacementpowerfactorforanon-linearloadsuchasaVSD,truepowerfactorcanbeapproximatedas:
ForaVSDwithaTHDIof45%,thenthetruepowerfactorwouldbe0.92.
Theabovearejustsomeoftheaffectsofharmonicsinthesupplynetwork.Anunfortunateside–effectofenergy-savingdevicessuchasfluorescentlightswithelectronicballastsandvariablespeeddrivesisthattheycreateharmonics.Someoftheaffectscanbemeasured,andtheircostlyimpactquantified.AnincreaseinRMScurrentandareductioninpowerfactorhaveadirectimpactonelectricityusage,efficiencyandplantlifetime.
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S = P2 + Q2 + D2 (2)
1 + (THiD)/1002
1TPF ≈ (3)
eColi Br i u M • J u n e 2010 36
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MiTiGATion MeTHoDsConsumersincorporateharmonicmitigationwithintheirbuildingsforvariousreasonssuchastonegateaproblem,tocomplywithaspecificationorwithlimitsimposedbyanelectricaldistributor.Thequestiononwhethermitigatingharmonicsimproveenergyefficiencydependsonvariousfactors,basedontheseverityofharmonics,thetypeofloads,thelimitsrequiredtomeet,andthemethodofmitigation.Iftheimprovementstoelectricityconsumptionoutweightheaddedlossesoftheadditionalharmonicmitigationequipment,thenfurtherimprovementscanbemadetoenergyefficiency.Areviewofcommonmitigationmethodsaresummarisedbelow,focussingonbalancedthree-phaseequipment(suchasVSDload).ItisassumedthataVSDhassomeformofintegralharmonicmitigationssuchasACorDCchokes.ThemitigationmethodsreviewedareadditionaltotheintegralfilteringpresentwithinaVSD.
Inelectricalnetworkswithmorethanonesupplytransformer,apossiblemitigationmethodinvolvesover-sizingthesupplytransformertoreducetherelativesupplyimpedanceandvoltagedistortion.Distributingthelinearandnon-linearloadstoreduceaconcentrationofharmonicproducingloadsononetransformercan“dilute”apotentialproblem.Thisformofmitigationcanoccuratthedesignstageofaprojectandwouldincuradditionalcostforlargersupplytransformers.
AddingimpedancetothefrontofVSD,suchasACreactors,reducesthecurrentdistortionandthisreducesthevoltagedistortionattheterminalsoftheACreactorcomparedtothevoltagedistortionattheterminalsoftheVSD.TheACreactoraddsavoltagedropbetweenthesupplyandVSD,whichassistsintheharmonicreductionbuthasanunwantedsideeffectofreducingtheinputvoltageandhence,theoutputvoltage,oftheVSD.TypicalvaluesofACreactorsrangefrom1to5%impedance,withtheequivalentlevelofvoltagedrop.Typically,THDIwouldreducefrom40%to28%asthereactorimpedanceincreasedforagivenVSDonagivensupply.CostislowrelativetothecostofaVSD.
PassiveharmonicfiltersconsistingofinductorandcapacitorcircuitscanbeinstalledupstreamfromasingleVSD,oranumberofparallelconnectedVSDs.Thesepassivefiltersconsistofseriesandshuntcomponents,tunedtoblocktheharmonicsproducedbytheloadfromgettingintothesupplynetwork.Theintegralcapacitorsabsorbtheharmoniccurrents.ThesefilterscanbespecifiedtoprovideacertainlevelofTHDIatfullload,withtypicalvaluesintherangefrom5%to10%.
Passiveharmonicfiltersarerobust,simpletoinstallandrelativelylowcostcomparedtomoresophisticatedmitigationtechniques;however,theydohavesomeshortcomings.TheharmonicperformanceofTHDIbetween5%or10%isattheratedloadofthefilter.Operationatreducedloadswillslightlyincreasetheharmoniccontent.Butthecapacitorswithinthefilterwillalsoprovideleadingpowerfactor,whichcouldbeaproblemongenerators.AnotherimpactofpassivefiltersistheycanincreasethevoltageattheterminalsoftheVSD.Ensuringthefiltershavebeentestedwith,orhavebeentunedtothespecificVSD,willprovidebetterperformanceandoperation.Thefilteringcapabilitycanalsobereducedwithincreasedphaseimbalanceandhighvaluesofharmonicvoltagepre-distortion.
Shuntpassivefilters,consistingofinductorandcapacitorscircuits,inessencepowerfactorcorrectioncapacitorscircuits,
canbetunedtofilterspecificfrequencies.Capacitorsactasharmonicsinks,andbycombiningthemwithdetuningreactors,afiltercanbedesignedtoabsorbspecificharmonicfrequencies.Shuntbranchestoremovethefifthharmonic,theseventhharmonicandsoon,canbeaddedasneeded.Precautionshouldbetakenwhenapplyingshuntpassivefiltersasthefilterswillabsorbharmoniccurrentsfromnotonlytheload,butthesupplynetworkaswell.Incorrectlydesignedfilterscouldleadtomalfunctionandresonance.
Anactivefilterconsistsofacontrolcircuit,capacitorbank,inverterandlinefilter.Currenttransformersmonitortheharmoniccurrentsinthesupplynetwork.Thecontrolcircuitdeterminestheharmoniccomponentandconfigurestheoperationoftheinvertertoinjectharmoniccurrentsincounter-phasetotheexistingharmonicsinthesupply.Thetheoryofoperationcanbecomparedtoaudiblenoisecancelling.
Flexibilityinoperationandinstallationmaketheactivefilterversatile.Itcanbeinstalledattheoffendingload,ataswitchboard,atthetransformer,inneworretrofitinstallations.Themodesofoperationofafiltercanconsistofoverallharmoniccompensation,individualorselectiveharmoniccompensationandreactivepowercompensation(displacementpowerfactorimprovement).Moderndesignshaveintuitivecontrolpanelsandoffersupplymeasurementsandauto-tuningofthemeteringcurrenttransformers,havevariousIPratingstosuitdifferentinstallationlocationsandcanwithstandhighambienttemperatures.Activefilterscanalsooperateathigherlevelsofpre-distortioncomparedtopassivesolutions.
Carehastobetakenwiththeactivefilters,astheinverterswitchingfrequencycanbeimposedonthesupply.Thiscouldleadtoharmonicfrequenciesbeyondthe50thandpossibleresonance.Well-designedfiltersincorporateautomaticallyvariableswitchingfrequenciesthatarenotconsistentlyatonefrequencyandcanlessenthepossibilityofresonance.Increasingthelinefiltercomponentoftheactivefiltertolessentheimpactoftheswitchingfrequencycanbedone,buttheharmonicinjectioncapabilityisreduced.Withmostdesigns,abalancemustbeachieved.
HArMoniC sTAnDArDs AnD BesT PrACTiCeRegulationofharmonicsexistsviastandards.Standardsforpowerqualityareutilisedandappliedbysupplyauthorities,whichthenapportionlimitstoeachconsumerconnectedtothepointofcommoncoupling,orPCC.ThePCCiswheretwoormoreconsumersareconnectedtothesamesupply.Standardsexistthatgovernlimitsofharmonicsathigh-voltage(HV)andmedium-voltage(MV)networksandalsoforlow-voltage(LV)systemsandLVproducts.
Forexample,AS/NZS61000.3.12:2006,Electromagneticcompatibility(EMC)–Limits–Limitsforharmoniccurrentsproducedbyequipmentconnectedtopubliclow-voltagesystemswithinputcurrent>16Aand<=75Aperphase[3],isapplicableforbalancedthree-phase-harmonic-producingequipmentsuchasVSDs.ThisstandardisapplicabletoeachindividualVSD,andmostreputablevendorshaveacompliantproduct.However,thisstandarddoesnotconsidertheimpactofnumerousVSDsconnectedinanHVACinstallation,forexample.
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Maximum Harmonic Current Distortion in Percent of IL
Individual Harmonic Order (Odd Harmonics)
Isc / IL <11 11≤h<17 17≤h<23 23≤h<35 35≤h TDD
<20 4.0 2.0 1.5 0.6 0.3 5.0
20<50 7.0 3.5 2.5 1.0 0.5 8.0
50<100 10.0 4.5 4.0 1.5 0.7 12.0
100<1000 12.0 5.5 5.0 2.0 1.0 15.0
>1000 15.0 7.0 6.0 2.5 1.4 20.0
Even harmonics are limited to 25% of the odd harmonic limits above
Table 2: IEEE519:1992, table 10.3 harmonic current limits for system voltages to 69kV.
StandardslookingatasystemlevelorPCClevelaremoreapplicable,encompassingthebuildingaswhole.InAustraliathereisAS/NZS61000.3.6:2001,Electromagneticcompatibility(EMC)–Limits–AssessmentofemissionlimitsfordistortingloadsinMVandHVpowersystems(IEC61000-3-6:1996)[2],andrecommendationIEEE519:1992,RecommendedPracticesandRequirementsforHarmonicControlinElectricalPowerSystems[4],arefrequentlyreferencedandapplied.
AS/NZS61000.3.6provideslimitsofharmonicvoltagedistortionataPCCforbothMVandHVconnections(andalsoLV)andisusedbysupplyauthoritiestoensurepowerquality.ThesupplyauthoritiestypicallyprovidespecificlimitsbasedonconsumedpowerforeachconnectiontoensureharmoniclevelsateachPCCarewithinthestandard.CompatibilitylevelsofvoltagedistortionforLVandMVconnectionsareshowninTable1.
IEEE519:1992isarecommendationthatprovideslimitsofcurrentdistortionandvoltagedistortion.Currentdistortionlimitsareprovidedbasedonthefactthatlowercurrentdistortioncreateslowervoltagedistortion.Atpresent,onlyonestateinAustralia,Victoria,utilisesIEEE519:1992forharmonicdistortioncontrol.
ThevaluesofcurrentdistortionbasedonvaluesshowninTable2,Limitsareplacedontotaldemanddistortion,TDD,anexpressionofharmoniccurrentdistortionasapercentageofthemaximumdemandcurrentatthePCC,andrangefrom5%upwardsbasedonsupplycapacity(shortcircuitcapacity).Thebiggerthesupply,themoreharmonicsitcanabsorb.
Projectspecificationsmustalsoprovidespecificlimits,butcaremustbetakentonotprovideunnecessarilystrictlimits,inflatingcostsbeyondwhatisnecessary.Forexample,placinglimitsof5%THDIoneachthree-phaseharmonicproducingloadcanensurelowharmonics.Yetthismaybewellbelowtherequirementsfromthesupplyauthority.Somepreliminaryworkatdesignstagecanprovideaworkingsystemthatisnotover-engineered.Knowingthelimitsfromthesupplyauthority,thecapacityofthesupply,thetotallinearandnon-linearloadandmaximumdemand,onecanestimatedistortionvaluesusingavailablesoftwaretools.
ConClusionHarmonicswithinapowersystemaregeneratedbynon-linearequipmentsuchasrectifiers,electronicballastlightingandVSDs.Mitigationisrequiredtomaintainpowerquality,improveenergyefficiencyandsystemutilisationandtoreducethepotentialfordevicefailure.
Odd harmonics not multiple of 3
Odd harmonics multiple of 3 Even harmonics
Order h
Harmonic voltage
%
Order h
Harmonic voltage
%
Order h
Harmonic voltage
%
5 6 3 5 2 2
7 5 9 1.5 4 1
11 3.5 15 0.3 6 0.5
13 3 21 0.2 8 0.5
17 2 >21 0.2 10 0.5
19 1.5 12 0.2
23 1.5 >12 0.2
25 1.5
>250.2 +
1.3*(25/h)
NOTE – Total harmonic distortion (THD): 8%
Table 1: AS/NZS 61000.3.6:2006, compatibility levels for harmonic voltages (in per cent of the nominal voltage) in LV and MV power systems.
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Mitigationmethodsvarydependingontheequipmentgeneratingtheharmonicsandthelocation,andthelevelofharmoniclimitsimposedonabuilding.Passivefilteringcanbeinstalledattheterminalsoftheharmonicgeneratingload(seriesfilters),oratthemainboard(shuntfiltering).Shuntactivefilteringprovidesvariousmethodsofoperationtomitigateharmonicsandimprovepowerfactorandflexibilityininstallationandretrofitapplications.
Harmonicstandardsexistforequipmentandsupplynetworks.Careshouldbetakeninimplementingappropriatestandards.Preliminarydesignworkforanewprojectcanincludesoftwareestimatesofharmoniclevelstoprovideaworkingsystemwithoutunnecessarylevelsoffiltering.
TheauthoracknowledgesthecontinualadviceandknowledgeprovidedbyGregersGeilager. ❚
referenCes[1]ABS,GuidanceNotesonControl
ofHarmonicsinElectricalPowerSystems,AmericanBureauofShipping,2006
[2]AustralianStandard:AS/NZS61000.3.6:2001,Electromagneticcompatibility(EMC)–Limits–AssessmentofemissionlimitsfordistortingloadsinMVandHVpowersystems(IEC61000-3-6:1996)
[3]AustralianStandard:AS/NZS61000.3.12:2006,Electromagneticcompatibility(EMC)–Limits–Limitsforharmoniccurrentsproducedbyequipmentconnectedtopubliclow-voltagesystemswithinputcurrent>16Aand<=75Aperphase
[4]IEEE519:1992,RecommendedPracticesandRequirementsforHarmonicControlinElectricalPowerSystems,InstituteofElectricalandElectronicEngineers.
About the authorTim Makris is an electrical engineer with extensive experience in the application of variable speed drives and harmonic mitigation equipment.
